U.S. Navy surface ships and submarines are presently constructed of structural steel plates that are rolled to four basic yield strength (kips per square inch or ksi) levels: ordinary or medium (aka mild) strength--(32-34 ksi); higher-strength or high-tensile strength (HTS)--(45.5-51 ksi); high-yield strength Grade HY-80 or HSLA-80--(80 ksi); high-yield strength HY-100 or HSLA-100--(100 ksi).
Previous engineering experience with the hull structural design of U.S. Navy aircraft carriers and cruisers provided the background to recognize the need for a 65-ksi steel that would offer naval architects, and design engineers of naval ship structures, a new cost-effective plate steel. The following factors were also to be considered:
A) It was believed that the use of a stronger steel in lieu of high-tensile steel (HTS) would allow a reduction in thickness of the plate and therefore the weight and still satisfy most design criteria and technical requirements.
B) Additionally, earlier experience with the development of HSLA-80 steel provided the background to recognize that a low-carbon steel would offer improvements in weldability compared to HTS plates.
C) There was also a need to provide better fracture toughness performance than that of HTS steels since the Navy is interested in improving ship survivability by preventing crack propagation in critical ship structures.
D) The physical characteristics of steel plate that must be met to provide the benefits for achieving a stronger, tougher, easily weldable steel plate having a high resistance to crack propagation are the following: a yield strength of a minimum of 65 ksi, a tensile strength of a minimum of 78 ksi, a minimum elongation in 8 inches and 2 inches for a plate up to and including 24 inches in width of 18% and 22% respectively and a minimum elongation of 16% and 20% for plate wider than 24 inches. Previously known plate steels could not provide these characteristics economically, if at all.
In the past, various modifications were made in the chemistry of the steel so as to meet selected requirements. However, the addition of expensive ingredients or the increase in the amounts utilized also increased the cost of the resulting products, often without corresponding increases in the steel plate performance characteristics.
Prior art U.S. Pat. Nos. 4,395,296 and 4,142,922 are typical of the attempts to produce a high strength steel plate with requisite toughness, but for one reason or another they failed to meet the desired specifications. U.S. Pat. No. 4,395,296, for instance, lacked the disclosure of the required chemistry for any steel plate produced. U.S. Pat. No. 4,142,922 discloses that manganese is not required in a significant amount and therefore limits the amount to 0.60% by weight, which is much too low for a low carbon steel such as for the present invention.